1. Field of Invention
This invention relates to the monitoring and detecting of optical power from a laser or laser source or from multiple lasers or laser sources such as in a laser array and, more particularly, to the monitoring and detecting accurately the front or “customer” end output power from a laser or lasers employing two photodetectors (PDs). As used throughout this description, the terms, “photodetector” or “photodetectors” may also be respectively abbreviated as “PD” or “PDs”. The invention has application to both CW (continuous wave) operated lasers with their outputs coupled to an external modulator or modulators and to directed modulated lasers (DMLs). As used herein, “modulated source” or “modulated sources” means either a directly modulated laser (DML) or a CW operated laser whose output is coupled to an external modulator (EML).
2. Description of the Related Art
For many years, semiconductor lasers have been mounted in modules or packages with their back facet outputs exposed to a photodetector, such as a PIN photodiode, also positioned within the module, so that the photodetector receives a small amount of the laser output from its back facet and provides a photocurrent which is converted into a voltage and, through signal processing, is employed to control the output power of the laser over time. It is well known to employ a feedback loop circuit to control the power output of a semiconductor laser by directing a portion of the light emitted by the laser to a photodetector to produce a current which is compared to a reference to provide an error signal, which, in some cases, may be integrated over time to produce a voltage or other representative value which is employed to control the bias current to the laser in order to maintain a constant output power from the laser over time. Such a system is shown in U.S. Pat. No. 5,123,024 where a communication semiconductor laser is also directly modulated, i.e., it functions as a DML. Also, such photodetectors can be integrated with the laser and in proximity to its output facet to monitor the output power of the laser as seen, for example, in U.S. Pat. No. 6,459,716. In other cases, a rear facet photodetector and front facet photodetector can be employed which receive a small percentage of the output power of the laser and the PDs can be employed in combination to provide for accurate power monitoring without the influence of optical crosstalk caused, for example, by ASE in connection with the pumping of an EDFA as seen, for example, in U.S. Pat. No. 5,847,856. Other power monitoring and control systems for lasers are shown in FIGS. 1A and 8A of U.S. Pat. No. 6,501,773 and FIG. 6 of U.S. Pat. No. 5,383,208. Since laser power versus current characteristics as well as laser operating wavelength change with laser operating or ambient temperature as well as with aging, these monitoring and control systems are employed to maintain the output power as well as the operational wavelength of the laser at a constant value. With an ambient temperature increase, the laser threshold increases and alters the output power at a given bias current. Also, the current threshold of the semiconductor laser increases with age to end-of-life and the power output decreases with age to end-of-life which is clearly illustrated in FIGS. 3 and 4 of U.S. Pat. No. 5,383,208, supra. Thus, it is common to detect the laser power with a photodetector and employ a closed loop feedback circuit to provide a laser bias current that provides the desired output power of the semiconductor laser over its lifetime.
It is also known in the art to employ low frequency tones for applications different from use in connection with direct power monitoring. For example, in U.S. Pat. No. 6,016,326, two tones of the same frequency, but out of phase by 180 degrees, are employed to maintain the bias of a direct modulated laser (DML) at the laser threshold. One pilot tone is applied to the data modulation drive current sources and the other pilot tone is applied to the laser bias current source. A photodetector detects a portion of the laser output and generates a photocurrent from the laser ASE which is used to alter the laser bias current to be at the desired laser threshold. Also, pilot tones have been superimposed on a data modulation signal employed to directly modulate the semiconductor laser to set the extinction ratio of the DML as disclosed in U.S. Pat. No. 5,850,409. Also, such low frequency tones have been employed for tagging different laser sources so that each source can be identified in a tapped portion of a multiplexed or optical channel group (OCG) signal comprising the combined modulated wavelength channel signals in the optical transmitter output. The tagging is for the purposes of pre-emphasis or power equalization across the array of discrete channel laser sources operating at different wavelengths as depicted in FIG. 13 of U.S. Pat. No. 6,271,945. In yet another application disclosed in U.S. Pat. No. 6,556,321, as seen in FIGS. 11 and 12, low frequency modulated tones in the kilohertz range are employed to carry maintenance data in an optical transmission network via one or more transmitter semiconductor lasers along with high frequency payload data in the gigahertz range.
What is needed, however, is a way to accurately determine the output power from the front facet output or “customer end” of a laser transmitter employed in an optical communication system or network.